can i use 73 duster lca for my bbp disc swap in my 65 valiant?

The only reason to change the LCA is to get sway bar brackets. If you do, the early A's had the sway bar brackets in a different place, so you would need a later sway bar to match, and I don't know if the vehicle width is the same (LCA bolt-bolt distance). While you can bolt an aftermarket sway bar to a non-bracket LCA (recall at the shock mount), you might consider welding on brackets. There was at least one FABO post of a home-made welded-on bracket that looked as good or better than factory.